Method of making high-density sintered chromium-bearing iron alloys

ABSTRACT

CHROMIUM-BEARING IRON ARTICLES AND PARTICULARLY CHROMIUM-BEARING STEEL ARTICLES SUCH AS STAINLESS STEEL OBJECTS ARE PRODUCED BY COMPACTING, REDUCING, AND SINTERING MIXTURES OF METAL COMPOUND POWDERS AND FERROCHROMIUM POWDERS. AT LEAST 35 PERCENT, BY WEIGHT, OF THE POWDER IS OF A PARTICLE SIZE LESS THAN 10 MICRONS.

Int. Cl. B22f 1/00 U.S. Cl. 75-211 15 Claims ABSTRACT OF THE DISCLOSUREChromium-bearing iron articles and particularly chromium-bearing steelarticles such as stainless steel objects are produced by compacting,reducing, and sintering mixtures of metal compound powders andferrochromium powders. At least 35 percent, by weight, of the powder isof a particle size less than microns.

BACKGROUND In US. patent applications Ser. Nos. 872,481, filed Oct. 30,1969, entitled Method of Making High Density Sintered Metal, and827,846, filed May 26, 1969, entitled Slip Casting, there is revealedthe surprising dis covery that dense metal objects free of cracks,internal defects, and surface irregularities may be obtained bycompacting, reducing, and sintering very fine-grained metal compounds.Particular success has been demonstrated in the production of fine-gagewire and thin wall tubing by first extruding plasticized metal-powdercompounds, reducing in a hydrogen-yielding atmosphere, and sintering theextruded and reduced product. Particular success has also beendemonstrated in the production ofhigh density slip cast articles byreducing in a hydrogen-yielding atmosphere and sintering.

The metal compounds are preferably those easily reduced, i.e.,characterized by having standard free energies of reaction with hydrogento form elemental metals of less than about kilocalories per gram atomof hydrogen at the reaction temperature. We have had particular successin reducing fine metal oxide particles-particularly iron oxideparticles.

A prerequisite for our success has been the use of powders having aparticle size distribution wherein at least 35 percent, by weight, ofthe particles are less than 10 microns in diameter. However, preferablythe mean particle size will be no greater than about 6 microns and atleast 25 percent of the powder has a particle size no greater than about2.5 microns. We have had particular success in utilizing iron oxideparticles wherein all of the particles are less than one micron indiameter as measured by Coulter counter.

When extruding fine-gage wire mils diameter or less) or thin-wall tubing(20 mils wall gage or less), none of the particles should have adiameter that exceeds about one third of the extrusion die orifice.

In the aforementioned patent application the fact is disclosed thatmetal alloys such as stainless steels may be fabricated from blends ofdiffering reducible metal compounds or powder mixtures of one or morereducible metal compounds or powder mixtures of one or more re-3,677,749 Patented July 18, 1972 ducible metal compounds and one or moremetals. The use of ferrochrome metal powders in combination with ironoxide in the production of chromium-bearing steel is illustrated.

THE INVENTION This invention relates to the discovery that surprisingsuperior chromium-bearing steel products substantially equivalent towrought steel are obtainable when using the process of theaforementioned patent application by utilizing ferrochromium as thesource of chromium.

Although Cr O powders may be blended with Pe O and reduced just prior tosintering, the time required for complete reduction of the chromiumoxide below the melting or sintering temperature of iron or nickel isexcessive. We have been able to make a successful product in this mannerin that we have been able to produce finegage stainless steel wirehaving a continuous chromiumbearing corrosion-resistant surface butshowing unreduced Cr O' particles and reduced but nondiifused chromiumin the core of the product. Although this wire exhibits surprisinglygood mechanical properties for having unreduced Cr O and is a useful andmarketable product, it is not the equivalent of conventional wroughtstainless steel either in respect to long time corrosion resistance ormechanical properties.

Other chromium compound particles 'which may be substituted for the Cr Oare CrO (chromic acid) and CrCl These materials are deliquescent, makingit diflicult to maintain a consistent water content in the plasticizedmass for proper compaction. However, of equal importance is the factthat some Cr O forms during the reduction of the Fe- O (or Fe O wherecompounds such as Cr0 or CrCl are present to provide a structure similarto that obtained when the source of chromium is Cr O \Additionally,where chromium metal compounds such as Cr O CrO Crcl etc., are reducedto particles of elemental chromium or Where elemental chromium metalpowders are used as the chromium metal source such elemental metal failsto migrate or diffuse into the iron or iron and nickel matrix at asufficiently rapid rate to effect a reasonably homogenouschromium-containing steel except perhaps at the surface of thecompacted, reduced, and sintered part.

We have found that powdered ferrochromium, particularly the commerciallyavailable grades of ferrochromium commonly used in the steel industryfor chromium additions to steel in conventional melting practices, areideal for providing chromium in the method of the present invention.

The commercially available ferrochromium alloys consist essentially ofbeneficiated and upgraded iron-chromium mineral deposits havingchrominum contents ranging from about 50 to percent, by weight, andcarbon contents ranging from a trace to 8.00 percent, by Weight. Thesignificance of these alloys is that the chromium is already in solutionin an iron matrix or perhaps it is more accurate to say that from 30 to50 percent, by weight, iron is in solution in a chromium matrix. In anyevent, we have found that such an alloy in powdered form mixed with ironoxide powder upon reduction and sintering forms a homogenous oressentially solutionalloyed chromium containing iron or steel.

Ferrochromium frequently contains impurities such as phosphorus, sulfur,silicon, manganese, etc. Such impurities may be present in proportionalamounts that will not unduly contaminate the chromium-bearing iron orsteel articles being produced. For example, when producing A.I.S.I. orS.A.E. Type 304 stainless steel, the sintered product should contain nomore than about .045 percent maximum phosphorus, or .030 percent maximumsulfur and accordingly a powdered ferrochromium should be selected andadded to appropriate iron and nickel oxide powders in amounts that willnot only yield from about 18 to 20 percent, by weight, chromium butwhich at such concentrations will yield no more than about .045 percentmaximum phosphorus and .030 percent maximum sulfur. Correspondingselection is readily used to limit the silicon, manganese, and carboncontents of the ultimate product.

The use of ferrochromium is a source of chromium metal for the alloyingof dense metal products made in accordance with the method of ouraforementioned patent applications has the added advantage of providingan easy means for supplying carbon to the ultimate product. Carbon, ofcourse, is an essential addition to steel and to meet A.I.S.I. of S.A.E.specifications for Type 400 series stainless steel or chromiumcontaining low alloy steels such as Types 4140, 4340, and 5140, it isnecessary to effect the addition of carbon. Commercially availableferrochromium alloy may contain a wide range of carbon contents so thatpractically any carbon contents can be easily established while meetingany of the desired commercial chromium contents.

The use of high carbon content ferrochromium alloys (1.00 percent carbonor greater) over low carbon content ferrochromium alloys is preferredbecause the high carbon content compositions are more easily comminutedto a fine particle size by mechanical means (crushing, grinding, etc.)due to its greater hardness and brittle characteristics. The particlesize of the ferrochromium will preferably be the equivalent of the ironoxide powder (at least 35 percent, by weight, under microns; preferablya mean particle size not greater than 6 microns and at least 25 percent,by weight, being under 2.5 microns; and optimumly all of the particlesbeing under 1 micron).

A method of producing a low carbon content product while using a highcarbon content ferrochromium consists of decarburizing high carboncontaining articles during the sintering step by providing a highmoisture to 40 F. dew point) environment. In this reaction oxygen fromthe moisture forms CO with the surface emerging carbon. This procedureis particularly advantageous in producing low carbon, thin-gage wireshapes or thin-Wall tubing since decarburization is effective throughoutthe cross-sectional area of the extruded, reduced, and sintered article.

It will, of course, be understood that other alloying additions may bemade either by including metal compounds of alloying metals in the ironoxide-ferrochromium powdered mixture or by including other alloyingmetal powders. In either event, the total particle size distribution ofthe powdered mixture must be within the above-recited parameters (atleast 35 percent, by Weight, under 10 microns; preferably a meanparticle size of 6 and percent, by Weight, being under 2.5 microns; andoptimumly all of the particles being below 1 micron in diameter). Theadvantages of the present invention relating to high density, goodsurface, and high mechanical properties are largely lost where excessivequantities of metal powders are employed. Further, the pyrophoric natureof fine metal powders make them hazardous to handle particularly in amixture of oxide particles. Consequently, it is preferable that thetotal quantity of metal powder (including ferrochromium) does not exceedabout 50 percent, by volume, of the mixture.

For example, we prefer to blend nickel oxide powder with iron oxidepowder and ferrochromium powder when we produce the Type 300 seriesstainless steels (essential- 1y l8 Cr-18 Ni-balance Fe). Such a mixturemay be blended with a plasticized or binder such as starch and water,extruded into fine-gage wire (or thin-wall tubing), reduced with atemperature range of from about 930 F. to 1200" F. in the presence ofreducing gaseous environment (hydrogen, hydrogen yielding, or CO) andsintered within the temperature range of from 1830" F. to 2450 F. toform a dense smooth surfaced austenitic steel wire which will meet mostspecifications for wire made of these grades of steel. High reducing andsintering temperatures and long time treatments such as are necessary toeffect reduction and diffusion of chromium compounds and chromium metalare unnecessary and the ultimate product exhibits a structureessentially free of unreduced metal compounds or undiifused metalparticles.

The preferred means of making further alloying additions depends, ofcourse, on the physical properties of the addition. For example, inproducing Type 316 stainless steel we may use elemental molybdenumpowder or molybdenum oxide powder (M00 whichever is most economical tomake the molybdenum addition. Where additions of silicon are to be madethese may be made by the addition of ferro silicon powder. Further,manganese may be provided through the use of ferromanganese.

Although chromium-bearing steel products made in accordance with themethod of the present invention closely approximate conventional wroughtchromium-bearing steel products, some improvement of density andcorrosion resistance may be effected by subsequent working. For example,the density and corrosion resistance of austenitic grades of stainlesssteel wire made in accordance with the method of the present inventionis measurably improved by subsequent drawing and annealing or sintering.

The use of hydrogen to provide the environment for reducing the metalcompound powders to elemental metal is a preferred embodiment of thepresent invention; however, we have found that other reducing materialsmay be employed. For example, we have found that the aboverecited metalcompounds and particularly iron oxide can be reduced by partially orwholly substituting carbon monoxide for the hydrogen reducingenvironment.

Any metal compound powders having particles of any general shape (i.e.,spherical, oblong, needles, or rods, etc.) and originating from anysource (i.e., ore deposits, ore concentrates, precipitates, etc.) may beemployed for compaction, reducing, and sintering in accordance with thepresent invention. The sintered article derived will possess asubstantially pore free structure, a smooth surface, and will exhibitdensities generally in excess of percent of theoretically completelydense material. We have, however, discovered that metal oxide powdersobtained by the process of spray drying a dissolved metal compoundprovides superior compacts (particularly extrusions) that reduce andsinter in a manner to provide objects of greater density and bettersurface and structural integrity than slips made of metal oxides fromother sources.

Spray drying of solutions containing dissolved metal compounds to effectmetal oxide powders is a well-known prior art procedure. For example,this method is utilized to regenerate hydrochloric acid picklingsolutions that have been used in the iron and steel industry to removemill scale and other forms of iron oxide from iron and steel products.The used aqueous pickling solution containing up to about 11 percent, byweight, free hydrochloric acid, and up to about 35 percent ferrouschloride is sprayed through a nozzle into a heated chamber (about 1000P.) where the ferrous chloride is converted into iron oxide andhydrochloric acid, as follows:

One version of the process is described in the article LiquorRegeneration Slashes Cost of Steel Pickling by Joseph A. Buckley,Chemical Engineering, Jan. 2, 1967, pages 56-58.

Regardless of the exact parameters or specific apparatus used, oxidesproduced as above described and particularly spray-dried iron oxides arebelieved to consist of minute hollow spheroids. The spheroids themselvescannot be used to make satisfactory compacts for reducing and sinteringin accordance with the method of the present invention and it is ourtheory that when fragmented the resultant powders produce a compact ofsuperior characteristics for use in conjunction with the method of thepres ent invention.

We believe that the fragmented spray-dried particles tend to agglomeratemaking accurate particle size determinations difficult. However, CoulterCounter measurements indicate that after three hours of dry ball millingthe powder is substantially all under one micron size (averagediameter).

By the term fragmented as it relates to the hollow spheroidal particlesobtained by the above-described spray drying technique, we mean thebreaking up of the hollow spheroids into smaller particles. Suchbreaking up is most conveniently accomplished by mechanical means suchas grinding. We have had particular success in ball milling suchspheroidal particles for periods of from about 1 to hours; however,other grinding techniques may be employed.

When practicing the preferred embodiment of the present inventionwherein spray-dried and fragmented metal oxides are utilized to producethe compact or extrusion, it will be preferred that the alloyingcompounds also be of the spray-dried-fragmented variety. Some advantagewill be experienced in utilizing any amount of spray-dried andfragmented metal oxides in the compact regardless of how small theproportion of these metal compound fragments are in relation to themetal compound particles; however, such advantages (green and sintereddensities and sintered structure) are not readily discernible where suchfragments do not constitute at least about 10 percent, by volume, of theparticles present.

Accurate particle size determinations of fine-grained powders arediflicult to obtain, particularly where the particle size distributionof such powders includes a fraction that is less than 10 microns indiameter. Such determinations are most difiicult where the particles areof nonuniform shape, For example, if the particles consist of crushed orground spheroids as is speculated in regard to ball milled spray driedHCl pickle liquor oxides many of the particles are likely to be of arelatively elongated or semicircular shape (sections of a hollowspheroid) so that it is diificult to determine actual diameter.Elongated particles will not pass through a screen having a mesh that isdesigned to accommodate a relatively symmetrically shaped particle ofequivalent mass. As a result particle size and particle sizedistribution measurements vary to a considerable degree for a givenpowder between the known methods and procedures for making suchdeterminations. For the purposes of the present specification and claimswe have used Coulter Counter Analysis to make particle sizedeterminations. In this system the particles are suspended in anelectrically conductive liquid and drawn through a small orifice. Acurrent is caused to flow through the orifice by means of two immersedelectrodes, one on each side of the orifice. As the particles flowthrough the orifice, the change of electrical resistance between theelectrodes is measured to determine particle size. Thus, the measure isone based on particle mass and is not affected by shape.

For the purposes of the present specification and the claims, allparticle size determinations and limitations are in terms of CoulterCounter measurements and shall include metal compound particles meetingsuch determinations irrespective of particle size determinations byother means.

Also, for the purposes of the present specification and claims the termscompacting or compaction" shall include slip casting.

EXAMPLES Fine-gage steel wire having analyses in substantial conformitywith stainless steel A.I.S.I. and S.A.E. Types 430, 431, 304, and 316were made in accordance with the method of the present invention. Theiron oxide was the spray dried byproduct of the closed-cycle HClpickling process. This oxide is Fe O and has a purity of 98.6 percent orbetter, the principal impurity after sintering being about 0.4 percentmanganese.

The ferrochromium had the following analysis:

C4.2 percent, by weight Cr68.l percent, by weight Feessentially thebalance When making Type 316 stainless steel, both powdered M00 andelemental molybdenum metal were employed.

Nickel for the Type 304, 316, and 431 grades was derived from reagentgrade NiO and NiCl All of the starting materials were dry ball milled ina steel-lined ball mill with an 8-inch inside diameter. A nominal chargeof 150 grams of powder per 1500 grams of steel balls was added to themill. The starting powders were less than 37 microns in size. Speed ofthe mill was r.p.m. The milling time was from 16 to 64 hours.

To grams of the mixture of ball milled metal compounds and ferrochromiumwere added 13.6 grams of a cooked water-starch binder consisting of 15grams of No. 34-1 Buffalo cornstarch and 100 ml. water, heated withstirring until gelled. Or, 100 grams of the metal compound-ferrochromiumpowder was mixed with 1.76 grams of dry pregelatinizied starch and 12ml. of water was added to this mixture.

The plasticized mixtures were then extruded into 18 mil diameterfilaments on a 75 ton hydraulic press at pressures of 2880' to 7200p.s.i. The die had a /z-inich 1diameter cavity, an 18 mil orifice, andwas l A-inches ong.

After drying in air at 300 F. for 30 minutes to remove excess water, thefilaments were heated rapidly to 1100 F. in an Inconel tube furnace witha dry hydrogen atmosphere (-40 dew point) and held at F. for 15 minutesto reduce the Fe O and NiO to iron and nickel. The filaments were nextheated slowly to 2200 F. as follows:

Holding Holding time at time at tempera- Temperatempera- Temperatureture ure ture F.) (min.) F.) (min.)

TABLE I Particle size Tensile properties distribution sintering timeSintered Composition Percent of Tensile Elongation Identi- Alloy Meaniameter, Theoretical strength, percent fieation type 25 p Max. [1 HoursF. mils Cr Ni Mo density K 5.1. in 1 in.

& Determined by Culter counter after ball milling mixture.

b Made with molybdenum metal powder.

Made with molybdenum trioxide.

Microexaminat-ion of specimens identified in Table I above as 1 showed arelatively porous structure; however, all of the other specimens showedvery little or only slight porosity. All of the specimens were ductileand could be bent into a tight When specimens identified as 6 were drawnto 3.8 mils diameter and resintered at 2000 F. for one hour, the tensilestrength was 104 K. s.i., the elongation was 41.4 (percent in 1 inch),the density was nearly 100 percent of theoretical, and themicrostructure revealed substantially no porosity.

Corrosion data on Types 304 and 316 wire are given in Table II below.The Type 304 (No. 5) wire, both as sintered and after cold drawing andresintering had good corrosion resistance. The Type 316 specimens 8 alsoshowed good corrosion resistance in the as-sintered condition. The Type316 specimen in which M00 was used as the source of molybdenum showedpoor corrosion resistance in the as-sintered condition; however, afterdrawing and resintering this specimen showed good corrosion resistance.

mix of iron oxide (spray dried by-product of the closed cycle HClpickling process), nickel oxide (NiO) and ferrochromium powder (C 4.2percent, Cr 68.1 percent, balance essentially Fe). The mixture wasbalanced to meet Type 304 specifications. The HCl iron oxide wascalcined at approximately 850 F. for eight hours and reground toeliminate excess acid. All of the starting materials were dry ballmilled for 48 hours in the steel-lined ball mill with an 8-inch insidediameter in the manner described above. The average particle size wasless than one micron. The slip was mixed by tumbling in a ceramic ballmill. It was necessary to make hydrochloric acid additions to lower thepH and render the slip castable.

A drain casting was made at a pH of 6.45. After drying and separationfrom the mold the casting was reduced at 1150 F. in a hydrogenatmosphere (held at temperature for approximately /z-hour) and insertedat 220 F. for three hours.

TABLE TIL-CORROSION RESISTANCE OF sTAINLlfiilsg lTtljiiggs WIRE PRODUCEDFROM OXIDE-FERROCHROMIUM Density Percent Corrosion Wire 0 rate, SampleAlloy diameter, Carbon theomils per identification type Condition nnlspercent G./ce. retical year Remarks As sintered 12. 5 0. O8 7. 2 91 7Drawn and resintered 9. 0 0. 08 7. 9 100 5 Conunerci 5-20 Rating in 70percent boiling nitric acid.

As sintered 12. 6 19 Produced from Mo powder. do 11. 8 0. 31 7. 8 98Produced from M00; powder.

Drawn and resintered 8. 5 0. 09 8. 0 100 11 Do. Commercial 6-20 Ratingin 70 percent boiling nitric acid.

B Determined according to ASTM Designation A-262-56T percent boilingnitric acid).

b Average of five 48-hour test periods. Completely dissolved in first48-hour evaluation. 4 Metals Handbook, Volume 1, page 75, ASM (1968).

The Wire specimens of Tables I and II above that were redrawn aftersintering were drawn from about 12 mils (as-sintered diameter) to about4 mils. This wire was drawn through a sequence of 14 diamond wire diesof the following sizes:

Type 430 stainless-steel wire could be drawn to 4-mils diameter withoutan intermediate anneal. Type 304 required intermediate anneals at 1 850to 2050 F. after drawing to 9 and 5 mils, and Type 316 wire requiredintermediate anneals at 1900 to 2100 F. after drawing to 11, 9, and 5mils.

After drawing, some wires were resintered at 2200 F. in dry, purifiedhydrogen to produce wire of nearly 100 percent density.

A Type 304 stainless steel slip was prepared from a The resultantsintered product exhibited a clean, smooth, crack-free surface and adensity of 92.3 percent of theoretical density.

We claim:

1. In a process for making chromium bearing articles from agglomeratesthat consist essentially of a mixture of particulate metal compounds,particulate ferrochromium, and a plasticizer or binder by compactingsaid agglomerate into a shaped compact, exposing said compact to areducing environment for a period of time disposed to elfect reductionof substantially all metal compounds reducible in said environment andexposing said compact to a temperature disposed to eifect sintering ofthe reduced metal particles and ferrochromium particles so as toincrease the density of said compact wherein the improvement comprises:

-(a) having at least 35 percent, by weight, of the particles of saidmixture being less than 10 microns in diameter; and

(b) reducing said compact at a temperature in the range from 930 F. to1200 F.

2. The method of claim 1 wherein said reducing environment is providedby at least one reducing agent selected from the group of hydrogen andcarbon monoxide.

3. The method of claim 2 wherein a major portion of said metal compoundsconsist of iron oxides and said chromium-bearing article consists of asteel article.

4. The method of claim 3 wherein the mean particle size of said mixtureof particles does not exceed about 6 microns and at least 25 percent, byweight, of said particles are less than 2.5 microns.

5. The method of claim 4 wherein said agglomerate is compacted by beingextruded through a die orifice to form an elongated-shaped article.

6. The method of claim 4 wherein said metal compounds consistessentially of iron oxide, said reduction temperature is from about 930F. to 1200" F. and said sintering temperature is from about 1830 F. to2350" F.

7. The method of claim 6 wherein said agglomerate is compacted by beingextruded through a die orifice to form an elongated-shaped article.

8. The method of claim 7 wherein said exrusion is a filament having adiameter of 20 mils or less.

9. The method of claim 6 wherein said agglomerate is compacted by beingslip cast.

10. The method of claim 4 wherein said metal compounds include a mixtureof iron and nickel compounds and said compounds and ferrochromium arepresent in proportions to meet the composition ranges of chromium,nickel, and iron for Type 300 series stainless steels when sintered.

11. The method of claim 4 wherein said iron oxides and ferrochromium arepresent in proportions to meet the composition ranges of chromium andiron for Type 400 series stainless steels when sintered.

12. The method of claim 8 wherein said metal com- 10 pounds consist ofmaterials which in combination with said ferrochromium are disposed toyield a composition upon sintering that will meet the essentialcomposition limits of Types 300 and 400 stainless steels.

13. The method of claim 12 wherein said filaments are subjected todrawing disposed to reduce the cross-sectional dimensions thereof aftersintering.

14. The method of claim 7 wherein the particles of said mixture ofparticles are substantially all under 1 micron diameter as determined bya Coulter counter.

15. The method of claim 5 wherein said elongated shape is tubing havinga wall thickness not greater than about 20 mils.

References Cited UNITED STATES PATENTS 2,175,850 10/1939 Patterson et al-211 2,315,302 3 1943 Volterra 7521 1 2,686,118 8/ 1954 Cavanagh 75-211FOREIGN PATENTS 557,950 6/ 1959 Canada 75--211 OTHER REFERENCES Hausner,Harry: New Methods for the Consolidation of Metal Powders, 1967; PlenumPress, pp. 3-4.

CARL D. Q UARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner U.S.Cl. X.R. 75-2100

